1. Field of the Invention
The present invention relates to a method and apparatus for detecting a molten metal level inside a container for melting metallic material in a case where a nonferrous metal having a low melting point, such as zinc, magnesium, or an alloy thereof, is melted to be subjected to injection molding in a liquid phase state.
2. Detailed Description of the Prior Art
It is necessary to detect molten metal levels for reasons including: the need to prevent the overflowing of molten metal stored inside a container for melting metallic material, such as a melting furnace; or the need to control operations wherein unmolten metallic material is fed into containers for melting metallic material.
Known means for detecting molten metal levels are: an indirect method using a photoelectric sensor or the like, wherein light is shed on the surface of molten metal, and the resulting reflected light is sensed to detect the molten metal level; and a direct method using a conduction sensor or the like, wherein an electrode is directly inserted in molten metal, and it is determined whether or not continuity exists between the electrode and the molten metal, thereby detecting the molten metal level.
However, in the case of either of the above-mentioned two methods, it is necessary to install a sensor in a high-temperature portion inside a container for melting metallic material, and therefore, any usable sensor is subject to restriction. Furthermore, if the container for melting metallic material is so constructed as to have an intricately complicated interior, or if the container for melting metallic material is so arranged as to have a very small interior, then it is difficult to detect the molten metal level by means of either a conventional photoelectric sensor or a conventional conduction sensor.
In particular, since the detection of a molten metal level by means of a photoelectric sensor is carried. out by sensing reflected light, this method cannot be used in the case of an arrangement wherein the interior of a container for melting metallic material is bent, thus preventing the surface of molten metal from being viewed from the position where the photoelectric sensor is installed. Even if the above-mentioned interior is so arranged to permit the photoelectric sensor to be used, the construction of the portion where the photoelectric sensor is installed becomes so complicated as to present a problem in that the cost of the sensor itself becomes high.
Moreover, in the case of the direct detection of a molten metal level by means of a conduction sensor, molten metal is prone to remain attached to the electrode portion located at the tip of the conduction sensor. The molten metal attached as above presents no particular problem in the event that the interior of a container for melting metallic material is large enough to permit a sufficient distance to be secured between the conduction sensor and the wall surface of the container. However, if the interior of a container for melting metallic material is small, then the distance between the conduction sensor and the wall surface of the container is inevitably short, and therefore, the electrode portion of the sensor and the wall of the container are prone to contact each other on account of the molten metal attached to the electrode portion, resulting in continuity being present at all times. Consequently, the conductivity sensor loses its function, thus presenting problems such as failure to carry out detection.
Furthermore, in addition to the above-mentioned methods for detection, there exists a method wherein inert gas is adopted as a detecting medium; the through-flow and the absolute value of the pressure of the inert gas introduced into molten metal are measured; the distance between the surface of the molten metal and the location through which the inert gas is introduced is determined on the basis of the value of the absolute pressure and the value of the pressure acting on the surface of the molten metal; and the molten metal level is electrically detected and adjusted on the basis of the above-mentioned distance. In the case of this method, the difference between the value of the absolute pressure and the value of the pressure acting on the surface of the molten metal is so minute that it is difficult to detect variations of the above-mentioned difference by means of a normal pressure gage, and therefore it is necessary to use a high-precision, costly pressure measuring instrument.
It is an object of the present invention to provide a means for solving the above-mentioned problems caused by conventional photoelectric and conduction sensors, wherein inert gas which is fed into a container for melting metallic material, for the purpose of preventing molten metal from being oxidized, is adopted as the detecting medium. It is yet another object of the present invention to provide a new method and apparatus for detecting a molten metal level inside a container for melting metallic material, wherein variations of a molten metal level are detected on the basis of variations of a liquid level inside a detector caused by a rise in the pressure inside a pipeline; and even if changes in the pressure inside the pipeline is minute, a very simple arrangement is capable of reliably detecting the rise of the surface of molten metal at all times without requiring any high-precision pressure measuring apparatus to be used.
For the purpose of achieving the above-mentioned objects, the present invention provides a method wherein communication is established by means of a gas-pipeline for inert gas between that portion of a container for melting metallic material which is above the molten metal surface inside the above-mentioned container for melting metallic material and that portion of a detector comprising a transparent tubular container which is above the liquid surface inside the above-mentioned detector, and variations of the liquid level inside the detector due to a rise in the pressure inside the gas-pipeline are detected as upward variations of the molten metal level.
Furthermore, the apparatus according to the present invention comprises: a gas-pipeline for inert gas whose open end is inserted in a container for melting metallic material, wherein the opening of the above-mentioned open end is caused to face into that portion of the above-mentioned container which is below a supply port and which is above the surface of molten metal inside the above-mentioned container; a detector comprising a transparent tubular container, in which the interior of the above-mentioned tubular container is partitioned with a transparent partition wall to constitute a double structure having an inner portion and an outer portion, a plurality of liquid chambers are thus formed, the cross-sectional area of each of the above-mentioned liquid chambers is equal to that of the other, communication is established between the lower portions of the above-mentioned liquid chambers, the interior of the above-mentioned tubular container is filled with a required quantity of detecting liquid consisting of a liquid material having a low specific gravity, the upper portion of the above-mentioned tubular container is sealed, the central liquid chamber serves as a measuring chamber, and a liquid level gage is provided in the upper portion of the above-mentioned central chamber: wherein that liquid chamber in the above-mentioned detector which serves as the measuring chamber is connected to the above-mentioned gas-pipeline to establish communication between that portion of the above-mentioned container for melting metallic material which is above the surface of the molten metal inside the above-mentioned container for melting metallic material and that portion of the above-mentioned measuring chamber which is above the surface of the detecting liquid inside the above-mentioned measuring chamber; communication is further established by means of a pipeline between that portion of the outside liquid chamber which is above the liquid surface inside the above-mentioned outside liquid chamber and that portion of the above-mentioned container for melting metallic material which is above the surface of the molten metal inside the above-mentioned container for melting metallic material; and variations of liquid levels inside both of the liquid chambers due to a pressure rise in the above-mentioned gas-pipeline can be detected as upward variations of the molten metal level.
Yet another form of the above-mentioned detector comprises: a plurality of liquid chambers consisting of a pair of transparent tubular containers disposed side by side, wherein communication is established between the lower portions of the transparent tubular containers; detecting liquid consisting of a liquid material having a low specific gravity which is disposed inside each of said liquid chambers; and a liquid level gage disposed in the upper portion of each of said liquid chambers, with the upper portion of each of the above-mentioned tubular containers sealed: wherein both of the above-mentioned liquid chambers serve as measuring chambers; one of the above-mentioned liquid chambers is connected to the above-mentioned gas-pipeline; communication is established by means of a pipeline between the rest of the above-mentioned liquid chambers and that portion of the container for melting metallic material which is above the surface of the molten metal inside the container for melting metallic material; and variations of liquid levels inside both of the liquid chambers due to a rise in the pressure inside the gas-pipeline can be detected as upward variations of the molten metal level.
In the case of such an arrangement, it is the most desirable that the inside cross-sectional area of one of the two liquid chambers be set equal to that of the other. However, in the event that the inside cross-sectional area of the liquid chamber connected to the gas-pipeline is set larger than that of the liquid chamber connected to the pipeline, then variations of the liquid levels in both liquid chambers can be easily detected. Moreover, in the event that a colored or pigmented liquid is adopted, then visual confirmation of variations of the liquid levels can be easily carried out.
Furthermore, according to the present invention, even if a slight increase takes place in the pressure inside the gas-pipeline, the surface of the. liquid in the measuring chamber reacts, resulting in the above-mentioned liquid surface lowering, while the surface of the liquid in the liquid chamber connected to the pipeline rises. Therefore, such a rise can be detected as a rise exceeding a set level of the molten metal surface.
As regards the arrangement, it is sufficient to connect the detector to the gas-pipeline for inert gas, which is a means for preventing oxidation, and to the pipeline for establishing communication with the interior of the container for melting metallic material. Consequently, the present invention can be applied widely without being restricted by internal constructions of containers for melting metallic material such as melting furnaces.